Force capability of a planar peristaltic Terfenol-D motor
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abstract
A magnetostrictive peristaltic motor, similar to an Inchworm motor, was designed, built, and tested, using Terfenol-D as the active element. Unlike the Kiesewetter motor, which uses a rod of Terfenol-D in a closely fitting cylindrical channel, in our design the Terfenol-D is in the form of a flat slab, held between two spring-loaded plates. With this configuration the motor is not affected by wear of the Terfenol-D surface. In order to avoid eddy current, we used laminated Terfenol-D. Operating at 1600 Hz with a peak field of 430 oersteds, the speed at no load was observed to be 12 mm/sec, which is 65% of the theoretical value (the peak strain times the peristaltic phase velocity), probably because the surface flatness of the Terfenol-D slab was only a few times smaller than the height of the peristaltic bumps. The measured stalling force was 90 newtons, corresponding to 350 kPa load stress in the Terfenol-D, only 3.5% of the peak strain times the stiffness of the Terfenol-D. This force was in almost exact agreement with a model which took into account the force from the spring-loaded plates (needed to avoid slippage), the transverse stiffness of the Terfenol-D slab, and the finite bending stiffness of the stator. The model can be used to design optimized motors with improved force capability. We tried using composite Terfenol-D, consisting of Terfenol-D particles in an epoxy binder, but the speed and stalling force were lower, perhaps because it did not have a very uniform distribution of particles, and was not machined as flat as the laminated Terfenol-D. The motor was driven by a specially designed 3-phase power inverter and digital controller, and the large reactive power inherent in this kind of motor was reduced by putting it in series with resonant capacitors.
